In my case, I decided to replace the fan because my cheap power supply's fan started making enough noise to drive me to distraction...
- Power supplies have dangerous voltages inside, even when completely disconnected. Capacitors on the line side usually retain their full charge even when unplugged, and can inflict a painful or even lethal shock. Please proceed only if you know what you are doing.
- Disassembling the power supply will void its warranty.
- Opening your PC may void its warranty, though I haven't come across such a computer so far. Also, fiddling with the insides may damage other components, so go ahead only if you are sure of yourself.
- 2011-05-02 : Corrected explanation of fan dimensions. Thanks to KanyonKris for the correction (see comment at bottom)